NEDSP 900 User Manual
9
Issue: 4
17. Suggestions for Module Use
The following section deals with the basic common practice requirements for integrating the
NEDSP900 module into a system.
A. Layout considerations
The NEDSP 900 board is designed to provide a plug-in solution for noise cancellation. The board
itself is designed to provide the best immunity from electrical noise and the best inter channel
isolation, while giving the user plenty of latitude in way to incorporate the board into their system.
It’s recommended that a good quality double sided PCB is used for the layout with good grounding
throughout any circuitry preceding or following the module. Any ground areas should be solidly
connected with all others to ensure good grounding system.
As previously stated, while the power pin is decoupled within the module, it is recommended that
further decoupling is employed between pins 1 and 2 of the module. A set of parallel capacitors
with values 10uF, 100nF and 10nF should be employed. If the supply rail is to be used for other
circuits, further decoupling capacitors of appropriate values should be employed locally at these
circuits as well.
In particularly noisy supply systems, use of individual ‘Pi’ filtering on the power supply lines should
also be considered to de-couple the more sensitive devices.
B. Audio Lines
When placing audio tracks to and from the module pins, try to make as much use of the way in
which the ground pins are provided to ensure isolation between the audio signals and the digital
control signals.
Connecting Microphones
Only channel 1 (Pin 11 Signal /Pin 10 or 12 Ground) supports very low level inputs such as those
provided by microphones. Almost any type of microphone may be used including low impedance
(Circa. 300R or less) magnetic devices or higher impedance (1k0 or more) Electret inserts.
In the case of magnetic microphones, many of these have very low output levels of between 2mV
and 5mV. With these signal levels you will probably need the highest gain available on audio
channel 1. This will require all the ‘Chl1 gain’ pins to be grounded to provide the 30dB of gain.
In the case of Electret microphones, these can produce up to 20mV and therefore lower gains may
be selected such as 21dB through to 27dB. Note that the module does not provide a bias voltage for
electret microphones and this will have to be provided as part of the circuit into which the module is
to be integrated. The manufacturer of the Electret module will usually provide the appropriate bias
requirements and how to meet them. The NEDSP audio input is DC isolated already, alleviating the
need for extra DC Isolation capacitors being required between the microphone output and the
module.
